The present invention relates to an apparatus for controlling the amount of intake air to an engine and, more particularly, to an apparatus equipped with a structure in which a controlled amount of auxiliary air is supplied so as to bypass the throttle valve.
In a known structure as disclosed in Japanese Utility Model Publication No. 18124/1984 or Japanese Patent Laid-Open No. 5856/1984, a throttle valve is installed in an intake passage to control the amount of intake air to an engine, and a bypass passage that bypasses the throttle valve has a bypass control valve. The output power of the engine can be varied at will by arbitrarily controlling the amount of intake air to the engine by the use of the throttle valve. When the engine is warmed up, the bypass control valve is controlled according to various engine parameters, such as the temperature of the engine coolant and the engine speed under unloaded condition, to achieve desired objects.
Various devices are installed in an engine compartment to enhance the performance of the engine and, therefore, large space is not left in the compartment. The bypass passage structure including the bypass control valve is detachably mounted to a throttle body which includes the throttle valve and forms the intake passage. The bypass passage structure and the throttle body form a compact unit. Of course, limitations are imposed on the arrangement of the various devices in the engine compartment.
Under these conditions, the throttle body is disposed horizontally, i.e., the intake passage extends horizontally. Also, the valve shaft of the throttle valve perpendicular to the intake passage is disposed horizontally. The bypass passage structure including the bypass control valve is mounted either above or below the throttle body. In case that the structure is disposed below the throttle body, the spatial arrangement is more advantageous than the case where it is disposed above the throttle body. In the former case, the bypass passage that bypasses the throttle valve in the intake passage normally extends vertically downward through the throttle body and opens at the bottom of the throttle body.
Fuel contained in the gas discharged from the engine, fuel contained in the circulating blowby gas, and moisture, carbon, oil mist, and other substances produced by combustion (hereinafter referred to as liquid adhesives) adhere to the inner surface of the throttle body. Especially, when the engine is not in operation, these adhering substances flow down the wall surface by the action of gravity and are collected in the lower part of the cylindrical wall surface. Then, they pass through the vertically extending bypass passage which opens at the bottom of the throttle body, and flow into the valve portion that controls the bypass passage. Eventually, they are deposited and caked on the metering portion of the valve seat and also on the sliding portion of the valve body. As a result, a metering error occurs and smooth operation of the valve body is impeded, so that the accuracy in controlling the amount of auxiliary air is deteriorated.